1. Field of the Invention
The present invention relates to medical devices and methods. More particularly, the present invention relates to implantable luminal prostheses and other medical devices which degrade in a body environment.
2. Description of the Background Art
Coronary artery disease is the leading cause of death in the industrialized countries around the world. It begins as the accumulation of atherosclerotic deposits in the walls of the major arteries which supply blood to the heart. As the deposits accumulate, normal blood flow to the heart is restricted. The heart has several compensatory mechanisms, which, to a point, can offset such diminished blood flow. Beyond these compensatory mechanisms, a number of well established pharmaceutical treatments have been shown to improve both symptoms and mortality in patients with mild to moderate coronary artery disease. However, as the disease progresses, its symptoms become more apparent, despite drug therapy. When the heart does not get enough blood, particularly during exercise or stress, advanced coronary artery disease is manifested as debilitating chest pain or angina. At this point, mechanical intervention is required to increase the amount of blood flowing to the heart.
Angioplasty is one of the most common interventional treatments for advanced coronary artery disease. Andreas Gruntzig performed the first percutaneous transluminal coronary angioplasty (PTCA) procedure. He advanced a catheter with a small balloon through the aorta and into a coronary artery with a partial occlusion. He then inflated the balloon, compressing the plaque against the arterial wall, and restoring blood flow to the heart.
PTCA has grown rapidly, and angioplasty catheters have become smaller and more maneuverable, allowing interventional cardiologists to access more difficult coronary blockages. However, restenosis, or reocclusion of the treated lesion, has plagued PTCA. Typically 30-40% of all patients have restenosis following PTCA.
Coronary stents were introduced in the mid 1990's to prevent restenosis. A stent is a small metal coil, slotted tube, mesh or scaffold structure that is placed in a coronary artery. It is a permanent implant that remains in the coronary artery following PTCA. The stent helps hold the artery open, improves the flow of blood, and relieves symptoms of coronary artery disease. Coronary stents were the first devices proven to reduce restenosis, dropping the rate of restenosis to 15-20%. Stents have since been used in the majority of PTCA procedures.
Conventional stents have taken two forms, balloon expandable stents and self-expanding stents. Both are typically made of metallic materials and may include a biocompatible coating. Such stents are permanently implanted into the human body by deploying them on or through a catheter. Such permanent implantation may increase the amount of intimal hyperplasia, thrombosis or other adverse medical effects. Coronary stents accomplish a lower restenosis rate of 15-20% post angioplasty compared to PTCA alone as a result of maintaining a higher acute gain post procedure.
Drug eluting stents, which elute drugs such as rapamycin and paclitaxel, were designed to further reduce intimal hyperplasia rates with stents. Such drug eluting stents incorporate metal or metal alloys with degradable or non degradable polymers which control release of the drug. The use of such drugs has further reduced the rates of restenosis as compared to stents alone.
The metals or metal alloys used for both conventional and drug eluting stents are intended to be biologically stable and remain in the body for the patient's life unless surgically removed at a later date along with surrounding tissue. Thus, these stents do not permit temporary placement within the body unless patient and surgeon are prepared to undertake a second procedure to remove the stent, which is difficult or impossible in most cases.
Although one of the primary functions of stenting is to provide mechanical support to the blood vessel wall and to preserve the lumen for blood flow, once the vessel wall heals the stent serves little or no continuing purpose. Further, the presence of a stent which remains mechanically rigid could potentially cause complications to the patient. It has therefore been desired to provide a stent which dissolves or degrades during or shortly after healing of the vessel or thereafter
There have been several attempts to make stents from biodegradable polymer materials such as poly-lactic acids (PLA). Such polymer stents, however, tend to provide less mechanical support for the vessel wall and therefore have to be substantially thicker than a comparable metallic stent. The thickness can reduce the available blood flow lumen and can cause undesirable biologic responses.
Recent attempts have been made to make metal stents which decompose in the body, as described for example in U.S. Pat. No. 6,287,332 B1 and U.S. Pat. No. 6,854,172 B2. See also US2004/009808 and WO 02/053202. Such degradable metal stents, however, often compromise strength, profile, and other desirable characteristics which are found in conventional metal stents.
For these reasons, it would be desirable to provide degradable devices that have improved physical and mechanical characteristics. In particular, it would be desirable to provide a stent or other luminal prosthesis which is degradable during and/or upon healing of the vessel or thereafter and which has features to reduce the risk of injury to the vessel or restenosis. It would also be desirable to provide localized and controlled release of a pharmacological agent from the stent or other device for the treatment of blood vessels and other body structures at the location being treated with the stent. Such pharmacological agents could minimize both restenosis and any inflammatory response towards the stent or other device and degradation products thereof. At least some of these objectives will be met by the aspects of the present invention.